U.S. Pat. No. 4,152,935 (Nissan) describes a device for measuring the mass flow of a fluid through a pipe made of an electrically non-conducting material. An ionized electrical field is introduced into the fluid with the help of a pair of electrodes. The first electrode of this pair is a peak projecting into the center of the fluid flow. High voltage impulses are applied to it. The second electrode of this pair is grounded. With reference to the fluid flow, it is located so as to be flush on the defining wall of the fluid flow on the same height as the first electrode, but at a distance to said first electrode. The fluid flow is comprised slightly downstream by means of a third electrode, which is also grounded. A grid electrode furthermore extends across the cross section of the fluid flow yet slightly further downstream. The time that passes until the ion cloud caused on the ionizing electrode by means of a high voltage impulse drifts through the fluid flow up to the grid electrode and causes a potential change against ground at that location is measured. The grounded electrode fixed between high voltage electrode and grid electrode, which comprises the fluid flow, serves the purpose of avoiding interfering undesired charges and discharges. Important disadvantages of this design are the requirement of an electrically insulating and thus non-metallic pipe, the fact that the measuring time becomes very long in response to slow flows, the fact that oppositely oriented flows can be measured only by an approximated duplication of the already considerable cost input, the fact that pulsating flows at which the amplitude of the motion is smaller than the measuring length cannot be measured at all and the fact that statements relating to the turbulence of the flow cannot be made.
U.S. Pat. No. 4,953,407 (General Motors) proposes an apparatus for measuring the gas flow in the common air supply pipe for an internal combustion engine, which is also based on the principle of the ion transport. To avoid interfering influences caused by the accumulation of polar molecules (H2O) on ions, the surrounding electrical field is kept so low that the thermal separation efficiency again compensates for the accumulation. Two configurations designed mirror-symmetrically with reference to a cross sectional plane of the fluid flow consisting of an acute high voltage electrode, which causes a corona, partial cylinder sleeve-shaped counter electrode and grid-shaped sensor electrode are proposed, a difference signal of which is detected. It is thus also possible to measure in both flow directions. For using the apparatus for measuring the air supplied to the internal combustion engines, it is disadvantageous that an electrically non-conducting pipe is necessary, that the apparatus is relatively long, that it cannot measure fast enough to correctly detect flows, which chronologically change rapidly and that it comes into saturation in response to high flow velocities and thus only supplies highly inaccurate values.
The publication “An ion-drag air mass-flow sensor for automotive applications” by Gerhard W. Malaczynski and Thaddeus Schroeder for the IEEE Conference 1989 points out that it would be highly advantageous to measure the air drawn in internal combustion engines in a cylinder-selective manner, but that this has the problem that the flow velocity in the individual suction pipes for currently known flow measuring devices would be too high. The object would be manageable with a configuration according to afore-described U.S. Pat. No. 4,953,407, if the distance between source electrode and collector electrode is chosen to be sufficiently large in adaptation to the maximal velocity to be measured. The disadvantage of this proposal is that it is oftentimes difficult to accommodate the required pipe lengths, that turbulences of the flow cannot be detected and mainly that the measuring sensitivity in response to low velocities becomes too low in response to the highest occurring velocities.
U.S. Pat. No. 3,242,729 describes the velocity measurement in a duct through which an electrolytic fluid flows. Three electrodes project into the duct at a distance behind one another. A current flow is established between the first two electrodes by means of a low voltage. The voltage is measured between the last electrode and the central electrode at a high measuring resistance. Said voltage provides information relating to the velocity of the fluid. The flow guiding mechanisms in an electrolyte are very different from those in an inherently non-conducting fluid, such as gas flows are in a normal case.
U.S. Pat. Nos. 4,056,003 and 4,167,114 show configurations where at least three grid electrodes, which preferably cover the entire flow cross section, are arranged in flow direction behind one another in a duct at a distance to one another. By means of voltage as compared to the second electrode and supported by radioactivity, a corona discharge is maintained on the first electrode. The electric charge arriving at the third electrode is measured. In addition to the disadvantage of radioactivity, the same disadvantages arise as already described above with reference to U.S. Pat. No. 4,152,935.
A similar principle, which also leads to the same disadvantages is also proposed in U.S. Pat. No. 4,136,564. A charge quantity introduced by means of high voltage is controlled to a constant variable per time. A grid electrode, which extends across the entire duct cross section, is fixed downstream. The charge quantity arriving at that location is measured; it provides information relating to the velocity of the fluid. The voltage required for continuously controlling the introduced charge provides additional information relating to the density of the fluid.
According to U.S. Pat. No. 4,163,389, charge carriers are introduced into the fluid at a first electrode pair by means of a corona discharge. An electric current is guided via the fluid duct via an electrode pair arranged downstream therefrom by means of applying a direct current. A conclusion relating to the flow velocity of the fluid is drawn from the chronological phase shift between a pulsing of the corona discharge and a pulsing of the current caused thereby at the electrode pair connected downstream. The disadvantages in turn are substantially the same as in the afore-mentioned U.S. Pat. No. 4,152,935.
U.S. Pat. No. 4,186,601 also proposes a similar functional principle, which in turn leads to the same disadvantages. Charge carriers are introduced into the fluid at a first electrode pair by means of a corona discharge. Either the voltage at this first electrode pair or at an electrode projecting into the flow downstream is triggered. The chronological course of the arriving signal is measured at an electrode, which is arranged even further downstream. The chronological shift between a shoulder of the trigger signal and a shoulder of the signal course caused thereby are used for drawing a conclusion relating to the velocity of the fluid.
There is an abundance of publications relating to how to measure the air drawn in internal combustion engines by means of hot-wire sensors. According to the underlying measuring principle, a wire around which the fluid flow to be measured flows, the electrical resistance of which is highly dependent on the temperature, is heated beyond the temperature of the gas flow. The cooling by means of the fluid flowing around is a measure for density and velocity of the fluid. Considerable disadvantages of this quite common method are that the same measuring result is generated in response to a negative flow velocity as in response to a positive flow direction, that the measuring time is too slow for rapid controls, that the temperature range in which the sensor can be used is relatively limited and that the sensitivity against destruction caused by the solid particles floating along in the flow is relatively high and that the sensitivity against impacts caused by moisture is also considerable. Such sensors are thus always used only for accumulation intake pipes of internal combustion engines and never for the discharge pipes, which lead to the individual cylinders. There is an abundance of tricky proposals relating to how to correct the falsification of the measuring result by means of fluid flows temporarily flowing back, for example by means of local flow smoothing or by processing additional information relating to the respective operating state of the engine by means of interpolation. DE 196 33 680 B4 is mentioned as an example.
Accordingly, it would be desirable to provide a sensor for the velocity measurement of fluid flows, which is faster and more robust as compared to known sensors and which encompasses a larger temperature application. The sensor may also correctly detect a reversal of the direction of the fluid flow.